When to get a new bat? Whenever you like, and as part of this forum, more frequently than your bank manager will be happy with...
Seriously, older bats were less dried, more heavily pressed and therefore as a result, tended to last a lot longer. By contrast, newer bats are generally a lot lower moisture content, pressed differently and tend to have greater rebound at the cost of longevity. If you are getting a new one, you can always ask for a custom maker to press it more heavily, but the guide that is generally given is that a new bat now will last for roughly 1,000 runs equivalent use in nets and matches.